It is known to measure the mixing proportion of two gases by introducing each gas into opposite ends of a horizontally disposed porous tube containing a movable ball. The position of the ball in the tube is proportional to the ratio of the gases. If the tube is optically transparent it can be readily calibrated for different applied gases to provide a visual indication of the mixed proportion between such gases. This principal has been adopted in the past in an attempt to provide an inexpensive method of visually indicating the ratio between two gases flowing into opposite ends of a porous tube. A mixing chamber is disposed around the porous tube for receiving and mixing the gases delivered to each side of the ball respectively. Adjustment of the ratio between the applied gases is accomplished by throttling at least one of the input gases. This will cause the ball to move and assume a new location within the tube representing the new proportion. As long as the total mixed gas flow remains substantially constant the position of the ball along the tube will provide a substantially accurate as well as visual measurement of the volumetric ratio of the gases in the total mixture. Moreover, even if the total mixed gas flow rate is changed to a new rate the throttling mechanism can be used to reset the position of the ball so that the tube will provide an accurate ratio reading without modifying the original calibration.
For most practical applications however, it is essential that the volumetric ratio between gases remain essentially constant even though the mixed gas flow demand changes. In electric welding, for example, it is required to shield the arc working area with a shielding medium which might be represented by a mixture of carbon dioxide and argon with the proportion of each gas in the mixture and the total mixed gas flow dependent upon working conditions and operator satisfaction. Thus, not only is it necessary for the ratio between the gases to be adjustable but it is also necessary for the total mixed gas flow to be adjustable and preferably under the manual control of the operator. For multiple torch operation the total mixed gas flow demand is dependent upon the number of torches in operation at any given time and the selected gas flow setting for each torch. Hence, for acceptable operation, the apparatus must be capable of providing the gas mixture desired and capable of maintaining the desired mixture under varying mixed gas flow demand conditions.